Video displays using cathode-ray tubes (CRTs) have been widely used for several decades, although many problems in CRT technology still remain. Picture size is still limited, making group viewing difficult. The actual display units have a picture size of at least 19" (measured diagonally) which is about the smallest "comfortable" size for family home viewing. However, these remain large and cumbersome, hovering ominously over the room, collecting dust, consuming valuable floor space and creating an aesthetic eyesore. Additionally, a television convenient to watch sitting up becomes inconvenient to watch from a bed. In addition to these mere inconveniences, health hazards from X-rays emanating from color sets, eye strain related to flicker rate, sharp color peaks produced by phosphors and the dangers of high voltage and possible picture-tube implosion remain problems which have not been adequately addressed.
Image quality problems of CRT-based video displays include color distortion, lowered resolution from the influences of the earth's magnetic field, convergence error, age or misadjustment and decreased resolution due to visual artifacts such as scanning lines, phosphor stripes, and phosphor dots which are inherent in all such TV displays and are particularly visible when viewing at close range. These visual artifacts provide a poorer image quality than images in movie theaters.
"Projection televisions" have been developed and commercialized in recent years. Although such televisions have solved the small viewing screen problem, other existing problems have been exacerbated and new problems have been created. Projection televisions are more expensive than standard, direct-view televisions and are more cumbersome, heavier and larger so that portability is impractical. Two types of projection television systems have become popular: one using three CRTs with projection lenses and the other using an oil film scanned by an electron beam.
The CRT-based system remains very dim, requiring a dimly-lit viewing environment and a costly special screen which provides a very limited viewing angle. The three CRTs produce images in the primary colors: blue, green, and red. The oil-based system, often referred to as an Eidophor System, has three "scanned oil elements" which have a relatively short life and uses an external light source. In either system, these images must be converged onto the screen to form one color picture. Due to the curvature of the lenses and variations in the performance of the circuits in either system, proper convergence is not easily achieved and sometimes requires up to a half hour of additional set-up time. If the projector or screen is moved, the convergence procedure must be repeated. The CRTs are driven with a high anode voltage to obtain as much brightness out of them as possible. Increasing the anode voltage further increases the X-ray hazard and lowers tube life and other problems associated with high voltage. The three tubes increase the danger of tube implosion.
Many attempts have been made through the years to solve the above-mentioned problems by using a "light valve" based system. This type of system uses an external light source which can be as bright as desired, with a light valve to modulate the light carrying the picture information. The research and experimentation to develop a workable light valve has been focused upon using different optical effects coupled with physical effects and finding or producing various materials to accomplish the desired effects in a light valve. With the exception of the oil scanning type of system, no other light valve system has proven feasible or economically viable.
Experimentation has also been performed on a laser system which scans out an image on a viewing screen in the same way an electron beam scans the image onto the face of a CRT. The laser system is much too large to be portable, very complex to use and maintain, extremely expensive, very dangerous and has proven too dim for large images.
The various light-valve system attempts have mainly utilized: crystals, such as quartz, Potassium Di-Hydrogen Phosphate, Lithium Niobate, Barium Strontium Niobate, Yttrium Aluminum Garnet, or Chromium Oxide; or liquids such as Nitro Benzene; or liquid crystals of the smectic or nematic type; or a suspension of particles such as iodoquinine sulphate in a liquid carrier. These and other similar materials have been used to capitalize on one or more optical effects including: electro-optical effects such as creating a rotated plane of polarization or altering the index of refraction of the material due to an applied electric field, magneto-optical effects using an applied magnetic field, electro-striction effects, piezo-optical effects, electrostatic particle orientation, photo-conductivity, acousto-optical effects, photochromic effects, laser-scan-induced secondary electron emission, and various combinations of these effects. Unfortunately, such light valves have proven impossible to manufacture inexpensively, in large quantities and with a large aperture and have often been toxic, dangerous and inconsistent in production quality.
In all light valves, different areas must be supplied different information so that a different amount of light would emerge through each area, adding up to a complete picture across the total beam of light. This requires the materials to be scanned by a laser or electron beam or for a tiny criss-cross of electrically conductive paths, i.e., a matrix, to be deposited on or adjacent the material to be addressed. In scanning beam systems, problems included outgassing, erosion of material and image information loss due to the bright and hot illuminating light. The electrical matrix system has proved difficult to engineer, requiring good conductivity characteristics with extremely fast switching circuits, which were impractical at the high voltages required to activate a given area of material. The most frequently used system (developed to address small areas) which has shown promise is often referred to as electronic multiplexing.
Electronic multiplexing only works with low-voltage requiring materials such as liquid crystals. With this method, all pixel addresses are x and y coordinates on the conductive grid. To activate a given pixel area a specific amount, different voltages must be applied to the x and y conductors so that, where they meet, they together exceed a threshold and modulate the area. A major drawback to such multiplexing is crosstalk, where surrounding areas are affected by the local electric field, causing false data to influence surrounding pixels. Crosstalk is also a problem with electron and laser scanned materials and reduces contrast and resolution as well as color saturation and accuracy.
Since these light valves have very little persistence and one pixel area is activated at a time, substantially less light passes through the screen to ultimately arrive at the viewer since all pixels are "off" most of the time. This characteristic wastes light, produces a dimmer image with poorer contrast, and generates more heat because of the brighter source necessary to compensate. High refresh rates are impractical because that would require faster switching times and faster responding material.
"Pocket TVs" are constructed today using the electronic multiplexing technique, but because the picture is small, the light source bright and the ambient conditions restricted, these defects are not very noticeable. However, when an image is projected, the defects are greatly magnified and become unacceptably noticeable as the large pixels form very noticeable squares and rows detracting from image quality. Contrast is then also noticeably very low--i.e., no "black" is possible. To further decrease contrast, the bright, hot lamp could heat up the LCD, causing a "hot spot" in the center of the image, spreading out in a Gaussian-like pattern. This lowers contrast further. Color rendition is also measurably poorer in such pocket TVs than with a CRT.
To address these and other problems associated with prior art video display devices, it is an object of the present invention to provide an adjustable size video image which can be very large, yet possess high quality and sufficient brightness to be visible in a normally lit room.
Furthermore, an object of the invention is to create a video display device which utilizes a specially constructed LCD light valve, an independent light source and optics for front or rear projection onto an internal or external screen.
Another object of the invention is to produce such a display with high resolution and contrast, and with more accurate color rendition, approaching that of a CRT, while reducing the strain associated with flicker sharp color peaks created by phosphors, and eliminating the appearance of stripes or pixels.
A further object of the invention is to produce a small, lightweight, portable system, having a long maintenance-free operating life, which is operable in conjunction with or without a large screen and can be mass produced relatively inexpensively.
Yet another object of the invention is to produce a system which requires no convergence or other difficult adjustments prior to viewing.
Still another object of the present invention is to produce a system which has no danger of emanating X-rays or tube implosion and operates with relatively low voltage.
An additional object of the invention is to produce a system which does not require a special screen, can be easily projected on a ceiling, and can be viewed comfortably at relatively wide angles.
A further objective of the invention is to produce such a system capable of three-dimensional projection.